Residential gateways are widely used to connect devices in a home of a customer to the Internet or any other wide area network (WAN). Residential gateways use for example digital subscriber line (DSL) technology that enables a high data rate transmission over copper lines. During the years, several DSL standards have been established differing in data rates and in range, for example ADSL, ADSL2, VDSL and VDSL2, which are referred to in this context as xDSL. Network operators, e.g. Internet service providers (ISP), are managing a large amount, up to millions, of residential gateways, also other devices such as routers, switches, telephones and set-top boxes, which are understood in this context as customer premises equipment (CPE) devices.
A wide area network providing xDSL services for a home network of a customer is schematically shown in FIG. 1: an ISP network 1 of the ISP is connected via an Internet router 7 with the global Internet 8 for providing Internet services to a residential gateway 2 and one or several client devices 3 of the customer. The connection between the ISP network 1 and the residential gateway 2 is provided by using a broadband remote access server (BRAS) 6 and a digital subscriber line access multiplexer (DSLAM) 5. The BRAS 6 may be provided within a Central Office of the ISP, together with the Internet router 7, whereas the DSLAM 5 is located close to the CPE device 2. The BRAS 6 and the DSLAM 5 are understood in the context of this application as a part of a Central Office Equipment of the ISP. Via xDSL connections 4, the residential gateway 2 and also home networks of other customers are coupled with the same DSLAM 5 of the ISP.
A Vectored VDSL2 system is a VDSL2 system that supports a self-FEXT (far-end crosstalk) cancellation technique. A “Vectored VDSL2 DSLAM” contains an array of collocated VDSL2 transceivers that are connected to a group of lines in a cable referred to as the vectored group. To support FEXT cancellation in the downstream direction to the customers, the transmission of the VDSL2 signals must all be coordinated as follows: all the signals operate with the same sub-carrier spacing, symbol rate, and cyclic extension length; in addition, the data symbols must be all phase aligned when transmitted on the line. Assuming the channel coupling matrices for the vectored group are known, downstream self FEXT cancellation is achieved as follows: the transmit data samples, all being transmitted phase synchronously, are processed through the equivalent of a pre-coder matrix. The pre-coder matrix effectively implements the inverse of a crosstalk channel matrix such that when the data samples are received at a far-end DSL receiver of a customer, the self far-end crosstalk injected in the cable by the transmitted data samples is effectively removed at the receiver input in the CPE.
The pre-coder matrix is constructed for all of the sub-carriers in the VDSL2 frequency bands for which self-FEXT is to be cancelled. Each transceiver in the vectored group of lines can be seen as processing one row of the pre-coder matrix taking data sample inputs from the neighboring transmitters for pre-coding. In other words, the crosstalk cancelling technique is achieved by injecting an “anti-signal” on each crosstalk-impaired line. In order to do that, a vector system needs a mechanism to estimate the crosstalk in order to derive the crosstalk coefficients, to invert the crosstalk matrix—each CPE will feedback to the DSLAM its own crosstalk coefficients, and the DSLAM will construct a matrix in which each row represents a CPE's feedback—and to calculate the “anti-signal”.
A more detailed description of the new G.vector standard, as specified by the ITU-T G.993.5 recommendation, and far-end (FEXT) and near-end crosstalk (NEXT) cancellation principles are described in V. Oksman “The ITU-T's New G.vector Standard Proliferates 100 Mb/s DSL”, IEEE Communications Magazine, October 2010, p. 140-148.
In Chenguang Lu et al, “A Fast Channel Estimation Method for Disorderly Leaving Events in Vectored DSL Systems”, ICC 2011-2011 IEEE International Conference on Communications, 6 pp., 5-9 Jun. 2011, a fast channel estimation method for disorderly leaving events in vectored DSL systems is described, which needs only a few error samples for providing channel tracking, in case the CPE device is unplugged or turned off.